A comment from BitTorrent on UTP:

Firon posted already about UTP:
******
uTP, the micro transport protocol. This UDP-based reliable transport is designed to minimize latency, but still maximize bandwidth when the latency is not excessive. We use this for communication between peers instead of TCP, if both sides support it. In addition, we use information from this transport, if active, to control the transfer rate of TCP connections. This means uTorrent, when using uTP, should not kill your net connection - even if you do not set any rate limits.
******

Just to re-iterate and offer a few more details (there's some pretty wild press reports popping up):
Firon described uTP completely accurately.
uTP is the result of a couple of years of work to try to make a Bittorrent protocol that works better on the internet.

The switch to uTP is at this point purely experimental, but the design objective (counter to some reports in the press) is actually to offer better congestion control than TCP offers, but maintain the same level of performance (speed).
Better congestion control is good for everyone – for users (VOIP, Gamers etc.) as well as for ISPs.
Same performance is what users have come to expect from their BitTorrent application – unless we can offer the same performance, then people will switch to a different BitTorrent client. (In reality we may be able to offer faster speeds too in many circumstances, but this is a byproduct and not the main objective.)

uTP is our UDP-based implementation of the BitTorrent protocol.
Normally BitTorrent is implemented on top of TCP which is the standard congestion control mechanism for the internet.
It so happens that the congestion control mechanism inside TCP is quite crude and problematic. It only detects congestion on the internet once “packet loss” has occurred – i.e. once the user has lost data and (probably) noticed there is a problem. The problems of TCP are fairly well known in technical circles, but it doesn’t get fixed as TCP is one of those protocols that is implemented in every OS, client and server, on the internet. Co-ordinating a giant upgrade is a very long process.

Because BitTorrent publishes the world’s most popular BitTorrent clients AND because these clients are talking mostly to each other (not to web servers), then we have an opportunity to detect end-to-end congestion and implement a protocol that can detect problems very quickly and throttle back accordingly so that BitTorrent doesn’t slow down the internet connection and Gamers and VOIP users don’t notice any problems. This is our objective.

This is great news for users of the internet and even for ISPs as it should mean that people make far more efficient use of internet bandwidth, but don’t over-use it to destruction. If uTP is successful, then internet congestion due to BitTorrent protocol could become a thing of the past. Of course there are many other applications that use the internet and they may also cause congestion, but we can only control what we do. Having said that, given that some press reports suggest that BitTorrent traffic constitutes half of all traffic on the internet, our technology might have a profound impact. We’re trying to do our bit to be responsible citizens on the internet.

While uTP is for now a proprietary BitTorrent protocol, we are also co-chairing an IETF group to address these issues. Hopefully that will lead to solutions that can be standardized and broadly adopted in due course.

Simon Morris
VP Product Management
BitTorrent, Inc.